1. Field of the Invention
This invention relates generally to the field of crystal oscillators and particularly to accelerating the crystal oscillator's start-up time, thereby reducing its power consumption while calibrating the crystal oscillating frequency.
2. Description of the Prior Art
Crystal oscillators are commonly employed in electronic circuitry and particularly used for generating a system clock for a synchronous system, such as a computer or communication systems. Crystal oscillators are generally expensive and even more so when they are of higher frequencies. Crystal oscillators are generally measured in parts per million (or ppm) as a measure of the accuracy of their oscillating frequency relative to a desired frequency. That is, for example, the larger the ppm, the less accuracy, hence, the lower the cost of the oscillator. Therefore, inexpensive oscillators, for use in high-performance prior art systems, often require fine calibration to meet, system-specified, small ppm requirements.
In addition, crystal oscillators generally yield slow ‘start-up times’, which is defined by the duration of time required for the oscillator circuit to generate a stable and desired frequency from the time of power on. Slow start-up times do not allow for rather frequent shut-off of the crystal oscillator when used in systems that enter, rather frequently, sleep or ‘deep sleep’ modes to reduce power consumption. That is, clearly, the more a circuit is in an inoperational mode, i.e. deep sleep, the less power the system will consume.
Therefore, the need arises for an inexpensive crystal oscillator to be used in an oscillator circuit capable of fast start-up time, accurate frequency calibration and low power due to frequent occurrences of ‘deep sleep’ mode.